util/kconfig: Uprev to Linux 6.6's kconfig

Upstream reimplemented KCONFIG_STRICT, just calling it KCONFIG_WERROR.
Therefore, adapt our build system and documentation. Upstream is less
strict at this time, but there's a proposed patch that got imported.

TEST=`util/abuild/abuild -C` output (config.h and
config.build) remains the same. Also, the failure type fixed in
https://review.coreboot.org/c/coreboot/+/11272 can be detected,
which I tested by manually breaking our Kconfig in a similar way.

Change-Id: I322fb08a2f7308b93cff71a5dd4136f1a998773b
Signed-off-by: Patrick Georgi <patrick@coreboot.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/79259
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Martin L Roth <gaumless@gmail.com>
Reviewed-by: Felix Singer <service+coreboot-gerrit@felixsinger.de>

util/kconfig/expr.h

@@ -275,7 +275,6 @@ struct jump_key {
 	struct list_head entries;
 	size_t offset;
 	struct menu *target;
-	int index;
 };
 
 extern struct file *file_list;